1. Field of the Invention
The present invention relates to a magnet assembly for use with an absolute linear displacement magnetostrictive position transducer. More particularly, the present invention relates to a precision permanent position magnet assembly used in conjunction with a magnetostrictive transducer to measure longitudinal displacement.
2. Background
Magnetostrictive displacement transducers, such as those manufactured by the MTS Systems Corporation are well known in the machine control industry. U.S. Pat. Nos. 3,898,555 to Tellerman, 4,952,973 to Tellerman, and 5,590,091 to Gloden and Sprecher, describe inventions using magnetostrictive technology. These measurement devices include a sonic waveguide assembly housed in a non-magnetic elongated tube with either integral electronic control circuitry or interfaces for the attachment of such circuitry. In normal operation, a short duration electric current pulse (nominally one to three microseconds) is applied to the head end of the magnetostrictively responsive waveguide portion of the waveguide assembly that creates a magnetic field surrounding the waveguide. The magnetic field of a permanent magnet positioned along the length of the active measurement region of the transducer interacts with the magnetic field surrounding the waveguide and causes a torsional strain wave response in the waveguide. The characteristics of the magnetostrictive waveguide material are such that first and second torsional responses propagate along the waveguide in each longitudinal direction away from the permanent position magnet at a gradient of approximately nine microseconds per inch, although the actual velocity varies with the composition of the waveguide material. In the usual application, the first torsional response travels towards the head or electronics end of the transducer where it is detected by a pick-up assembly including a sensor bound to the waveguide and in the MTS product by a tape surrounded by a pickup coil which is magnetically biased by a stationary permanent bias magnet. The sensing tape and pickup coil combination, or mode converter, converts mechanical, strain wave energy into electrical energy. A resulting electric signal is amplified and processed by an electronic circuit appropriate for the user's application. It is desirable to maximize the amplitude of the electric signal derived from the first strain response such that the electric signal-to-noise ratio of the transducer is also maximized. Any noise of whatever kind and from whatever source present in the electric signal has the potential to cause erroneous measurements and is, therefore, undesirable.
By electronically measuring the time period between the instant the electric current pulse is applied to the waveguide and the instant the first torsional response is detected at the pickup assembly and scaling this measurement by a known constant, in this case the velocity of propagation of the first torsional response along the waveguide, the longitudinal position of the permanent position magnet can be very precisely and very repeatably determined. In the usual application, this measurement is then used as a position detection means for controlling the operation of manufacturing equipment.
The second torsional response travels away from the head end of the transducer where the pickup assembly is housed and thus towards the other end of the waveguide, commonly referred to as the "damp end." This is typically an unwanted response that if not attenuated will be reflected back into the active measurement region of the transducer by the mechanism used to mount the waveguide into the waveguide assembly and housing, resulting in erroneous signal measurements. Therefore, the damp end of the transducer typically includes a braided, rubber, or other suitable damping material to reduce the amplitude of the second torsional response to a level that is difficult to detect and does not interfere with the normal operation.
Magnetostrictive transducers are used in many industrial applications. Certain applications require that the active measurement region be as large as possible in relationship to the overall length of the transducer assembly. Thus, it is often desirable to make the head and damp ends of a transducer assembly as short as possible so that measurements can be made in areas with limited space for mounting and operating transducers. The maximization of the active measurement region is hampered by such a requirement that the permanent position magnet be permitted to be in close proximity to the sensing tape and pickup coil, which are magnetically responsive by design. As the permanent position magnet is moved closer to the head end, this magnet's naturally occurring flux lines increasingly traverse the sensing tape, pickup coil, and flux lines of the bias magnet. The intersection of the flux lines of the permanent position magnet and the flux lines of the bias magnet causes a distortion or modulation in the normal patterns of the flux lines of both and causes an unpredictable change in the resulting electric signal. This change correspondingly results in less accurate position measurements when the permanent position magnet is near the head end of the transducer. The unpredictable changes in the electric signal diminish the accuracy of the overall system. Therefore, it is an object of the present invention to move the permanent position magnet closer to the tape, coil, and bias magnet, thereby maximizing the active measurement region.
A permanent position magnet assembly that appears similar to the present invention is disclosed in U.S. Pat. No. 5,514,961 to Stoll et al. However, the construction and object of the Stoll device are significantly different from the present invention. In the Stoll patent, the North-South alignment of the individual magnets comprising the assembly are aligned such that the North-South axes are parallel to the waveguide assembly. The object of Stoll is to provide a magnet assembly that, when used with a magnetostrictive transducer, causes the transducer to be substantially less susceptible to variations in the distance between the magnet assembly and the waveguide assembly. Some machine installations by design are unable to accurately control the positioning of the magnet assembly relative to the waveguide assembly. Stoll teaches a device that spreads the flux lines over a wider longitudinal surface of the waveguide assembly than is the normal case with a conventional magnet assembly. This arrangement causes the magnetic flux lines surrounding the magnet assembly to intersect the waveguide assembly at a lower field strength gradient than is exhibited by a conventional magnet assembly, and at a much lower field strength gradient than is exhibited by the present invention. While the Stoll invention achieves the object stated by Stoll, it is worse than a conventional position magnet assembly for the object stated for the present invention, and is therefore significantly worse than the precision permanent position magnet assembly that is the subject of this application.
It is a further object of the present invention to permit operation of a position magnet assembly within close proximity to the head end of the transducer.
It is another object of the present invention to miniaturize the size of the inner diameter of the opening in the permanent position magnet assembly.
It is another object of the present invention to lower jitter.
It is another object of the present invention to reduce tap sensitivity.
It is another object of the present invention to increase position accuracy detection.
It is another object of the present invention to raise the service temperature in which the invention may be used.
It is a further object of the present invention to permit longer transducers to be produced.